This invention relates to a process and apparatus for calibrating glass tubes.
Glass tubes can be produced by a continuous tube-drawing process in the case of nominal widths up to about 400 mm, by a discontinuous blowing process in the case of larger nominal widths up to about 600 mm, and by centrifugal forces between 600 and 1000 mm. The sections of glass tube produced by blowing or centrifuging are subsequently joined together by fusing to form glass tubes of the desired length.
The glass tubes produced by the abovementioned processes have considerable tolerances in outside diameter, which may be up to about .+-.1 mm in the case of small diameters (.ltoreq.50 mm) and of the order of up to .+-.5-6 mm in the case of larger diameters (.apprxeq.600 mm). The glass tubes produced by the centrifugal process have, in any case, a conical shape inherent in the method of production.
For many industrial applications, in particular for fabricating plants utilizing glass apparatus, it is necessary to provide glass tubes or sections of glass tube of precisely defined outside diameter. It is consequently an increasingly frequent occurrence in glass apparatus construction, in particular for composites having a wide variety of corrosion-resistant materials, such as for example PTFE or other fluoroplastics, high-alloy special steels, Hastelloy, titanium, tantalum, graphite, inter alia, that such composites have to be made, for example in the form of internal fittings for glass apparatuses. As a rule, these fittings must be sealed, and this requirement, on account of the poor tolerances attainable, heretofore has continued to be accompanied by problems.
Examples of such glass apparatuses which may be mentioned in particular are shell and tube heat exchangers and falling film evaporators. These essentially comprise a multiplicity of evaporator tubes which are arranged parallel to one another within a shell, and are open at their respective ends in the bores of end plates of metal, where they are individually sealed off from the outer space by means of O-rings. While the bores and also the O-rings have predetermined exact dimensions, the evaporator tubes may exhibit tolerances of about .+-.1 mm (with the outside diameters .apprxeq.40 mm). The sealing problems occurring in the assembly of these glass apparatus are therefore considerable.
In the prior art, various processes for the production of glass tubes with small dimensional deviations in the outside diameter have been proposed. Some of these proposals concern shaping measures which are taken while the drawing process is still going on. Others concern reshaping processes which are carried out on the finish-drawn, blown or centrifuged glass tube or section of glass tube.
German Patent Specification 2,229,164 and German Patent Specification 2,430,428 describe processes for the production of glass tubes having close outside-diameter tolerances during drawing, the still plastic tube leaving the drawing zone being brought to the desired outside diameter by means of suitable shaping tools, such as for example rollers or plates. These processes have the disadvantage that the shaping tools bearing against the outer circumference of the glass tube leave undesired pressure points on the outer wall of the glass tube on leaving the shaping zone, to the detriment of the outside-diameter tolerance. If an inadequate number of shaping tools arranged radially around the tube are used, the glass tube, in the worst case, may even be given an out of round cross-section. In German Patent Specification 2,430,428, for example, tolerances in the outside diameter of 0.2-0.3 mm are indicated over a tube length of only 20 mm.
German Offenlegungsschrift 2,807,596 discloses a process for the outer calibration of sections of glass tube of large nominal widths, according to which a section of glass tube clamped at both ends is brought to softening temperature in a deforming zone and the softened glass in the deforming zone is pressed against an outer calibrating face by means of a shaped part moved inside the section of glass tube along the path of a helical line with respect to the said section. Even in with this process, adequately close tolerances cannot be accomplished, the publication itself describing dimensional deviations on the order of tenths of a millimeter. Aside from being restricted to large nominal widths, a further disadvantage of the process is that the length of the calibrated section of glass tube is also restricted due to the risk of sagging in the softened region.
Consequently, at present the only possibility which remains for the production of a glass tube or section of glass tube of virtually any nominal width with a precisely defined outside diameter requires cylindrical grinding and polishing. However, like every "material-removing" machining, this damages the glass surface, as a result of which the strength of the component is considerably impaired. In addition, cylindrical grinding is a very laborious process.